hold q and select "Spawn Ragdoll"

spawn 100 ragdolls

hold q again and select "Play Mode"
then press e and select "Force Push"

then go where you placed the ragdolls
and then left click, you should unlock it.

if you didn't unlock the achievement, you probably didn't spawn a lot of ragdolls. or you didn't see the achievement pop up on the right bottom corner.

check steam and see if you have it